elaborate on the protocol... does it filter out certain frequencies necessary for a modem communication?
I tried something similar once without a direct cable though and with earth lines. I took two phones, one hooked up to the land line and one hooked up to my notebook computer (with a 9 v battery spliced in between one of the wires on the notebook and phone to create potential...). I then held the headsets of the two phones to each other (one upside down) and hit dial on the notebook and it connected at 9.6 kbps!